Bimodal lithium transition metal based oxide powder for use in a rechargeable battery

ABSTRACT

A bimodal lithium transition metal oxide based powder for a rechargeable battery, comprising: a first lithium transition metal oxide based powder, either comprising a material having a layered crystal structure consisting of the elements Li, a metal M and oxygen, wherein the Li content is stoichiometrically controlled, wherein the metal M has the formula M=Co 1−a M′ a , with 0≤a≤0.05, and wherein M′ is either one or more metals of the group consisting of Al, Ga and B; or comprising a core material and a surface layer, the core having a layered crystal structure consisting of the elements Li, a metal M and oxygen, wherein the Li content is stoichiometrically controlled, wherein the metal M has the formula M=Co 1−a M′ a , with 0≤a≤0.05, wherein M′ is either one or more metals of the group consisting of Al, Ga and B; and the surface layer consisting of a mixture of the elements of the core material and inorganic N-based oxides, wherein N is either one or more metals of the group consisting of Mg, Ti, Fe, Cu, Ca, Ba, Y, Sn, Sb, Na, Zn, Zr, Si, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Sc, Ce, Pr, Nd, Gd, Dy, and Er; the first powder having an average particle size (D50) of at least 15 μm; and a second lithium transition metal oxide based powder having the formula Li 1+b N′ 1−b O 2 , wherein 0.10≤b≤0.25, and N′═Ni x Mn y Co z A d , wherein 0.10≤x≤0.60, 0.30≤y≤0.80, 0.05≤z≤0.20 and 0≤d≤0.10, A being a dopant, the second powder having an average particle size (D50) of less than 5 μm, and preferably less than 2 μm.

This application is a National Stage application of InternationalApplication No. PCT/EP2013/074961, filed Nov. 28, 2013, which claims thebenefit of European Application No. 12197350.7, filed Dec. 14, 2012.

TECHNICAL FIELD AND BACKGROUND

The invention relates to a bimodal lithium transition metal based oxidepowder used as positive electrode material in a Li-ion rechargeablebattery, composed of two or more lithium transition metal oxide powders.

Rechargeable lithium batteries have many advantages compared to otherbattery systems. They show high energy density, high voltage, absence ofmemory effect and good cycle stability. Currently two of the majordrawbacks are problems related to ionic conductivity of cathode andelectrolyte, and the lack of safety of the charged battery.

The ionic conductivity of cathode materials is low. Thus liquidelectrolytes—which have much faster transport rates for lithium—areused. The electrolyte fills a network of interconnected pores, rangingfrom the cathode over the separator to the anode. The best liquidelectrolytes (e.g. salt dissolved in water) have an electrochemicalstability window at low voltage, whereas the lithium ion batteryoperates in a high voltage window. Thus electrolytes with anelectrochemical stability window at high voltage are needed. Suchelectrolytes are Li salts (such as LiPF₆ and LiBF₄) dissolved in organicliquids solvents, and typical examples for the liquid solvents arelinear or cyclic carbonates like propylene carbonate or diethylenecarbonate. These electrolytes have a relatively low ionic transportrate. The transport rate is still much higher than those of the cathodematerials but much less than for water based electrolytes. These factsillustrate that the ionic transport rate across the electrode is anissue in sé. In a rechargeable Li battery the electrode thickness isdetermined by the liquid electrolyte properties. Without going intodetails—the relatively low ionic conductivity of the organic solventsand certain transport properties of binary electrolyte (electrolytedepletion) limits the thickness of the electrodes. If the current is toohigh or the electrodes are too thick, then a mechanism—calledelectrolyte shutdown—limits the capacity at high discharge rate.Electrolyte shutdown is a property related to the binary liquidelectrolyte. An ionic transport within the solid cathode material ismuch slower, but the shut-down mechanism does not happen in the cathodematerial.

In order to achieve an acceptable rate performance Li ion batteries aremade of electrodes which (1) contain enough porosity (to be filled withelectrolyte in the final battery), and (2) need to be sufficiently thin(meaning low loading of active material (mg/cm²)) to allow a reasonabletransport of lithium across the electrode. Typical porosities are >12vol %, often 15 vol %, whereas loadings of 15-20 mg/cm² are typicalvalues. Porous, relatively thin electrodes are obtained by therelatively expensive ‘thick film technology’. As the ionic transport ismuch faster in the electrolyte than in the solid, there is a naturallimit for increasing the density of the electrodes. If the porosity istoo low then not enough electrolyte is present to support a sufficientfast ionic transport. Thus it would be highly desirable to develop acathode material which has a high ionic transport rate so that some ofthe Li transport across the electrode happens via the solid particles.In this way higher current rates can be applied.

The thickness of the electrodes and the porosity could be lowered, whichresults in an increased energy density of the lithium battery, becausemore active material fits into the confined volume of the battery. Orthe electrodes can be prepared thicker (but still support a high rate)and the porosity can be decreased. No cathode material has yet beenreported whose ionic transport rate approaches those of liquidelectrolytes.

Currently, due to a lack of safety of a charged battery, Li metal cannotbe used as anode. In general, anodes which contain extractable lithiumare dangerous to handle and are difficult to process. As a result thelithium needs to be supplied by the cathode, which potentially limitsthe choice of cathodes. The cathode typically is a lithium containingintercalation material. In intercalation materials lithium can beelectrochemically reversibly extracted and reinserted. Presently onlylithium transition metal oxides (or phosphates) are used as cathodes inrechargeable Li ion batteries. In the charged battery a delithiatedtransition metal oxide is in good contact with the organic electrolyte,as the latter fills the pores between the particles. If the batterybecomes “unsafe” (for example by external damage or heating) then achain of reactions can be triggered. A main reaction—very muchdetermining the safety of the real battery—is the reaction between thedelithiated cathode and the liquid electrolyte. This reaction isbasically the combustion of the solvent with oxygen from the chargedcathode. We will call it CCE (charged cathode-electrolyte) reactionwithin this invention. Batteries with less, or without organicelectrolyte would potentially be much safer because no CCE reaction canhappen. Such batteries are not available, because the rate performanceof the battery is too low, as was discussed above.

Carbon based anodes have been widely applied in rechargeable lithiumbatteries. A typical charge capacity Qch (lithiation of the anode) is360 mAh/g and a typical discharge capacity Qdc (delithiation of theanode) is 330 mAh/g. Thus a typical anode charge efficiency is330/360=91.7%. It is convenient to consider the irreversible capacityinstead: Qirr=1 minus charge efficiency, or Qirr=(Qch−Qdc)/Qch. Arechargeable lithium cell contains cathode and anode. The bestutilization of the reversible capacities of anode and cathode—yielding agood cell balancing—is achieved if the charge efficiencies match. Ifnot, an excess of cathode or anode material is needed, which excess doesnot contribute to the capacity of the lithium battery. Moreover thecharge efficiency should be matched not only at slow charge/discharge,but also at fast discharge.

In the following discussion we focus on Li batteries with very highenergy density. Very high energy density can be achieved by cathodeshaving either one or (preferably) both of a high volumetric density anda high specific reversible discharge capacity.

High volumetric density is easily obtained with relatively large, denseparticles. LiCoO₂ (LCO) is a very preferred material and can obtain highelectrode density. This applies especially to LiCoO₂ as described inWO2012-171780. LiNiO₂ based materials also allow for relatively highdensity electrodes as well. Such particles can only be applied in abattery if the Li diffusion constant of the positive electrode issufficiently high. If Li diffusion is too slow then the diffusion pathwithin the particles needs to be shortened, which can be achieved byreducing size and increasing intra particle porosity, thus ultimatelyresulting in nano-structured (high surface area and meso porous) cathodematerials. It is practically very difficult or even impossible toachieve high density with nano structured cathode materials.

A high specific capacity can be achieved with high lithium and manganesecompositions—also called HLM, being Li—Mn—Ni—O₂ with Li:M>>1 andMn:Ni>>1—cathode materials. They can be understood as a solid statesolution of Li₂MnO₃ and LiMO₂ whereM=(Ni_(1/2)Mn_(1/2))_(1−y)Co_(x)Ni_(y). x>0 signifies Co doped HLM.These compounds are sometimes considered to be nano-composites. A strictdistinction between the compounds is not possible because anano-composite—as the composite size decreases towards atomic scalebecomes a solid state solution. Undoped HLM cathode materials have avery high capacity—up to 290 mAh/g. The 290 mAh/g is typically achievedafter several activation cycles at a voltage of 4.8V and discharge to2.0V. These HLM cathode materials generally have very poor electronicconductivity and slow lithium diffusion, and therefore are prepared asnano-structured powders, making it very difficult to achieve a highelectrode density. After activation undoped HLM cathodes need to becharged to high voltage (at least 4.5-4.6V) otherwise their capacitiesare not sufficiently high. At these high voltages, surprisingly, HLM cancycle in a stable manner with little capacity fading.

The cathode materials mentioned before—LiCoO₂ (LCO) and HLM—are notmatching the anode charge efficiency well. LiCoO₂ as described inWO2012-171780 can have a very high charge efficiency of about 99%, evenat high rate, which is much higher than that of typical anode materials.This high charge efficiency is also obtained with large particles havinglow surface area. Even these large particles show a high rateperformance and very high charge efficiency, also at fast rate. Contraryto this, HLM has a low charge efficiency, which decreases dramaticallyif the rate is increased. Even submicron sized HLM cathode materials(with a D50 of 0.5-0.9 μm) with high surface area show poor rateperformances and low charge efficiencies at fast rate.

Even if different materials exist that provide either high volumetricdensity or high specific capacity, there is the need to develop onematerial that has both characteristics, and at the same time has a highionic conductivity and enables to operate in a rechargeable battery in asafe manner, and well balanced with the anode material.

SUMMARY

The current invention discloses a cathode material which combines theabove mentioned properties. Viewed from a first aspect, the inventioncan provide a bimodal lithium transition metal oxide based powder for arechargeable battery, comprising:

a first lithium transition metal oxide based powder, either comprising amaterial A having a layered crystal structure consisting of the elementsLi, a metal M and oxygen, wherein the Li content is stoichiometricallycontrolled, wherein the metal M has the formula M=Co_(1−a) M′_(a), with0≤a≤0.05, and wherein M′ is either one or more metals of the groupconsisting of Al, Ga and B;

or comprising a material B having a core and a surface layer, the corehaving a layered crystal structure consisting of the elements Li, ametal M and oxygen, wherein the Li content is stoichiometricallycontrolled, wherein the metal M has the formula M=Co_(1−a) M′_(a), with0≤a≤0.05, wherein M′ is either one or more metals of the groupconsisting of Al, Ga and B; and the surface layer consisting of amixture of the elements of the core material and inorganic N-basedoxides, wherein N is either one or more metals of the group consistingof Mg, Ti, Fe, Cu, Ca, Ba, Y, Sn, Sb, Na, Zn, Zr, Si, Nb, Mo, Ru, Rh,Pd, Ag, Cd, Sc, Ce, Pr, Nd, Gd, Dy, and Er; the first powder having anaverage particle size (D50) of at least 15 μm; and

a second lithium transition metal oxide based powder having the formulaLi_(1+b)N′_(1−b)O₂, wherein −0.03≤b≤0.25, andN′═Ni_(x)Mn_(y)Co_(z)A_(d), wherein 0.10≤x≤0.60, 0.30≤y≤0.80,0.05≤z≤0.20 and 0≤d≤0.10, A being a dopant, the second powder having anaverage particle size (D50) of less than 5 μm, and preferably less than2 μm.

In one embodiment 0.15≤x≤0.30, 0.50≤y≤0.75, 0.05≤z≤0.15 and 0.10≤b≤0.25,preferably 0.18≤b≤0.25. In another embodiment, the first powder consistsof large dense particles having a D50 of at least 15 μm, preferably atleast 25 μm, and most preferably at least 30 μm. In another embodiment,the bimodal lithium transition metal oxide based powder has a ratio ofthe D50 value of the first powder to the D50 of the second powder of atleast 3:1 when the first powder comprises material B, and at least 5:1when the first powder comprises material A or B. In a furtherembodiment, the first powder has an electrical conductivity of less than10⁻³ S/cm, preferably less than 10⁻⁴ S/cm, measured under a pressure of63.7 MPa. D50 is the median of the particle size distribution measuredby laser diffraction.

In another embodiment, the bimodal lithium transition metal oxide basedpowder has a ratio of the D50 value of the first powder to the D50 ofthe second powder is at least 3:1 when the first powder comprisesmaterial B, and at least 5:1 when the first powder comprises material Aor B. In a further embodiment, the first powder has an electricalconductivity of less than 10⁻³ S/cm, preferably less than 10⁻⁴ S/cm,measured under a pressure of 63.7 MPa. D50 is the median of the particlesize distribution measured by laser diffraction.

The bimodal lithium transition metal oxide based powder may have a1^(st) cycle discharge capacity of at least 220 mAh/g of activematerial, when used as an active component in a coin cell which iscycled between 2.0 and 4.6 V vs. Li⁺/Li at a discharge rate of 0.05 C,and preferably 0.1 C, at 25° C. In one embodiment, the first powder hasa first cycle charge efficiency of >96% when cycled at 3.0-4.6V, at 25°C., with a current of 16 mA/g at 25° C. in a standard electrolyte (1 mLiPF₆ in EC/DEC 1:2); and the second powder may have an activationvoltage characterized by a charge capacity of at least 290 mAh/g whencharging from 3.0 to 4.8V with a current of 80 mA at 25° C., in astandard electrolyte (1 m LiPF₆ in EC/DEC 1:2). Further features ofembodiments of the bimodal lithium transition metal oxide based powderare listed in the claims.

Key aspects of the embodiments of the present invention are:

1) The cathode powder allows for low porosity electrodes. In oneembodiment the cathode powder consists of a mixture of large denseparticles with low surface area and submicron sized small particles withhigh surface area. In another embodiment of the invention the lowporosity is achieved by very large particles of suitable shape.

2) For the bimodal powder embodiments the requirements for the large andsmall particles are different from each other. Therefore the compositionof large and small particles is different. In one embodiment of theinvention the large particles consist of a voltage stable LCO basedcathode material, and the small particles consist of HLM.

3) The LCO based material has been optimized to obtain very fast solidlithium diffusion rates and to be able to cycle with stability at highvoltage. In one embodiment the LCO based material cycles stably at 4.5V(versus Li/Li+) after a first charge at 4.6-4.7V. The optimization ofthe LCO based cathode material is related to a low electronicconductivity and a lithium to transition metal ratio of 1.0. The highvoltage stable compounds are further disclosed in co-pending applicationWO2012-171780, which is incorporated by reference and in its entiretyherein.

The process for manufacturing this kind of high stability compounds is a‘double firing’ process and runs as follows:

providing a first mixture of a first Co-comprising and a firstLi-comprising precursor powder, the first mixture having a Li to Comolar ratio >1.00,

sintering this mixture in an oxygen comprising atmosphere at atemperature T₁ of at least 600° C., thereby obtaining a Li-enrichedLiCoO₂ compound;

providing a second Co-comprising precursor powder,

mixing the Li-enriched LiCoO₂ compound and the second Co-comprisingprecursor powder, and

sintering the second mixture in an oxygen comprising atmosphere at atemperature T₂ of at least 600° C.

In one embodiment, the first mixture has a Li to Co molar ratio of 1.01to 1.12.

In another process embodiment, the step of providing a secondCo-comprising precursor powder comprises the substeps of:

providing a third Co-comprising precursor powder,

providing a second Li-comprising precursor powder, and

mixing quantities of the third Co-comprising precursor powder and thesecond Li-comprising precursor powder so as to obtain the secondCo-comprising precursor powder having a Li to Co molar ratio that isbelow 0.9, and preferably below 0.5.

In one embodiment, the second Li-comprising precursor powder is alithium carbonate powder. In another embodiment, the third Co-comprisingprecursor powder is free of lithium. In still another embodiment, thefirst Co-comprising and the second Co-comprising precursor powders areselected from the group consisting of cobalt oxide, cobaltoxy-hydroxide, cobalt hydroxide, cobalt carbonate and cobalt oxalate.Also the third Co-comprising precursor powders may be selected from thegroup consisting of cobalt oxide, cobalt oxy-hydroxide, cobalthydroxide, cobalt carbonate and cobalt oxalate.

4) The HLM based cathode material has been optimized to

(a) have high volumetric capacity in mixtures with said LiCoO₂

(b) have large reversible capacity and

(c) be fully activated after only one activation cycle at 4.6-4.7V

In one embodiment this performance has been achieved by doping HLM with5-20 mol % of cobalt.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Voltage profile (capacity of first charge as function of chargevoltage) of HLM with (a) 11 mol %, (b) 5 mol % and (c) 0 mol % Co.

FIGS. 2A, 2B, 2C: HLM rate and stability tests after different types ofactivation

FIGS. 3A, 3B, 3C: electrochemical testing of high voltage stable LiCoO₂

FIGS. 4A, 4B, 4C: Coin cell testing of mixtures of HLM and high voltagestable LiCoO₂

FIG. 5: typical SEM micrograph of dense high voltage LiCoO₂.

FIG. 6: Summary of coin cell testing of dense high voltage LiCoO₂.

FIG. 7: Summary of coin cell testing (cycle stability) of 50 μm LCO.

FIG. 8: Result of DSC measurement of charged electrodes (a) without and(b) with 1.16 mg of electrolyte added

FIG. 9: Micrograph of the LiCoO₂ having a D50 of 100 micrometer

FIG. 10: Micrograph of the HLM cathode having a PSD D50 of 8 micrometer.

FIG. 11: Voltage profile of the sample M05 (left) and sample M20 (right)cycles at C/20 rate (cycle 1) and C/10 rate (cycle 2) between 4.6 and2.0V at 25° C.

FIG. 12: DSC heat profiles of the (1) electrolyte free (2) 9%electrolyte containing and (3) 46% electrolyte containing electrodes.

DETAILED DESCRIPTION

It was mentioned before that there is a need for Li batteries with veryhigh energy density, that can be achieved by cathodes having preferablyboth of a high volumetric density and a high specific reversibledischarge capacity. Also, as the cathode's charge efficiency shouldmatch well with the charge efficiency of typical anode materials, amixture of LCO and HLM should be an ideal material. In one embodiment, away of mixing to achieve especially high densities is the bimodalapproach, where two powders are mixed. One powder consists of relativelylarge, dense particles, whilst the other powder has small particles.These small particles may be nano-structured. The small particles caneasily be pressed into the voids between the large particles. Obviouslythis approach will reduce the porosity of the electrode and allows toimplement nano structured cathode materials without loosing density.Since the low porosity reduces the transport rate in the liquidelectrolyte, it is compensated by a high transport rate in the solidmaterial.

Moreover, the bimodal mixture can be applied practically to lithiumbatteries if the operating voltages of LCO and HLM are adjusted to eachother. HLM requires an “activation charge” of typically 4.8V. Thisvoltage is generally conceived to be too high, as it was assumed thatneither LiNiO₂-based, nor LiCoO₂-based materials can cycle stablybetween 2V and 4.5-4.6V. In the present invention, a technique isdisclosed to lower the voltage of the activation cycle of HLM and/or toincrease the operating voltage of the LCO.

In one embodiment, the invention makes use of the advantages of the highvoltage stable LiCoO₂ that is disclosed in co-pending applicationWO2012-171780, describing a lithium metal oxide powder for use as acathode material in a rechargeable battery, consisting of a corematerial and a surface layer, the core having a layered crystalstructure consisting of the elements Li, a metal M and oxygen, whereinthe Li content is stoichiometrically controlled, wherein the metal M hasthe formula M=Co_(1−a) M′_(a), with 0≤a≤0.05, wherein M′ is either oneor more metals of the group consisting of Al, Ga and B; and the surfacelayer consisting of a mixture of the elements of the core material andinorganic N-based oxides, wherein N is either one or more metals of thegroup consisting of Mg, Ti, Fe, Cu, Ca, Ba, Y, Sn, Sb, Na, Zn, Zr andSi. There could also be more elements doped in the surface layer, suchas 4d elements and Rare-earth metals, like Er, Nd and Nb. This materialhas a reversible electrode capacity of at least 200 mAh/g, preferably210 mAh/g and most preferably 215 mAh/g when used as an active componentin a cathode which is cycled between 3.0 and 4.6 V vs. Li⁺/Li at adischarge rate of 0.1 C, and preferably at 1 C, at 25° C. It also has a1 C rate capacity fading value below 60%, preferably below 40% and mostpreferably below 30%. The capacity fade rate—expressed in %—at 1 C per100 cycles is calculated as follows: (1−(DQ32/DQ8))×100/23.(DQn=discharge capacity of cycle n).

The cathode material of the present invention has the followingproperties:

-   (1) Electrodes having a low porosity allowing for    -   (a) the use of less liquid electrolyte providing better safety    -   (b) a high content of active material providing high volumetric        and gravimetric capacity and energy density.    -   To achieve low porosity electrodes the cathode may consist of        particles which have a bimodal size distribution. In this case        the small particles will fit well into the voids between the        large particles.-   (2) Electrodes having a high reversible capacity. A high reversible    capacity allows to decrease the electrode thickness at constant    capacity, so the total porosity decreases. It is the total porosity    (total volume of pores in the electrode) which matters for safety    and not the porous fraction (pore volume/electrode volume).

3) The cathode material allows for very high ionic transport rate in thesolid phase. This allows achieving a good rate performance even in thecase of low porosity electrodes because solid lithium diffusion can actas a shortcut between the narrow liquid diffusion paths within theelectrolyte filled pores. For an efficient solid shortcut, if thecathode powder has a bimodal particle size distribution at least thelarge particle fraction has a high solid Li diffusion rate.

-   (4) The cathode material has a charge efficiency which matches well    the charge efficiency of typical anode materials.-   (5) The cathode material has a high gravimetric and volumetric    capacity and energy density. It performs well (rate performance,    cycling stability, . . . ) within the operating window of the    lithium battery.

Regarding the safety of the battery, a schematic example for a CCEreaction goes as follows—for LiCoO₂ as cathode material: the battery ischarged, meaning that Li is extracted from the cathode by the reaction:

$\begin{matrix}{{LiCoO}_{2}\underset{- {x{({{Li}^{+} - e^{-}})}}}{\rightarrow}{{Li}_{1 - x}{CoO}_{2}}} & (1)\end{matrix}$

Charging in this case needs high energy, since a potential of ≅4V isneeded to extract the lithium. Therefore the charged cathode isthermodynamically highly unstable. The equilibrium phases of charged LCO(in air) are LiCoO₂+Co₃O₄. The following reaction is thermodynamicallystrongly favored:Li_(1−x)CoO₂→(1−x)LiCoO₂ +x/3Co₃O₄ +x/3O₂  (2)

Thermodynamic estimations show that this reaction has a very high ΔG(free enthalpy change) but only a small ΔH (enthalpy change). As aresult not much heat evolves.

However, the situation is different in a real battery, where organicelectrolyte is present. The evolved oxygen will combust a part of theelectrolyte creating large amounts of heat. The electrolyte will alsoreduce Co₃O₄ to CoO. As an example of organic electrolyte we useethylene carbonate, and assume, for simplicity, full combustion:5/2O₂+C₃H₄O₃→3CO₂+2H₂O  (3)

Schematically the CCE reaction in a real battery can be written as

$\begin{matrix}\left. {{{Li}_{1 - x}{CoO}_{2}} + {\frac{x}{5}C_{3}H_{4}O_{3}}}\rightarrow{{\frac{3x}{5}{Co}_{2}} + {\frac{2x}{5}H_{2}O} + {\left( {1 - x} \right){LiCoO}_{2}} + {x{CoO}}} \right. & (4)\end{matrix}$

This equation tells us about the limitations of the CCE reaction:

-   1) Decrease x: less de-lithiated cathodes create less heat because    less oxygen is evolved to combust the electrolyte. Decreasing x or    the amount of LiCoO₂ is not meaningful because this will reduce the    capacity of the battery.-   2) Decrease the electrolyte: if it would be possible to make a Li    ion battery where the electrolyte content is lower than x/5 (x state    of charge in mol Li) per mol of LiCoO₂ then safety will improve    because not enough electrolyte is present to complete the CCE    reaction.

If the lithium transition metal oxide contains Mn or Ni instead of Co,this will not so much change the evolved heat of the combustion of theelectrolyte, but can change the kinetics dramatically, becausedelithiated Li_(0.5)CoO₂ (with Co in 4 valent state) is much morereactive than delithiated Li_(0.5)Ni_(0.5)Mn_(0.5)O₂ (where the 4 valentNi is “diluted” by much more inert Mn⁴⁺). Much less reactive means thatit is more difficult to start the CCE reaction (the battery needs alarger external heat or damage). Additionally if Mn is present theneventually less electrolyte is combusted because some 3 valent Mn (e.g.in Mn₃O₄) remains, so less oxygen is delivered (and less electrolyte iscombusted). Within this reaction we will—in our examples—focus onLiCoO₂, but the same conclusions are valid for other cathode materialsas well.

A very important aspect of cell safety is to lower the amount ofexothermic heat generated if a cell gets unsafe. At relatively lowtemperature (about 200-300° C.) the charged cathode starts to react withthe electrolyte. The charged cathode is in an oxidized state, so it isable to release oxygen, which combusts the electrolyte. The combustionreaction contributes a large amount of the heat when a battery getsunsafe. The higher the extracted capacity, the more the cathode isoxidized, and the more oxygen is supplied to combust the electrolyte.Obviously, if less electrolyte is present than needed to consume alloxygen then less combustion will happen and the safety of the batteryimproves. Reducing the electrolyte below a certain critical amount(where amount of oxygen releasable from the cathode and amount ofelectrolyte are in balance to allow full combustion) will improvesafety. The critical amount increases with increasing extracted capacityin what is called the electrolyte cathode balance.

The pores in the electrodes must be filled with electrolyte otherwisethe battery will have a poor performance. From a safety point of view itis desired to achieve a small porosity as much as possible, below the“critical porosity” which corresponds to the critical electrolyteamount, defined by the electrolyte cathode balance. So it is desired todecrease the porosity of the electrodes (and separator). However, thereis a limit because state of the art batteries need electrolyte tofacilitate the fast lithium diffusion in the liquid phase. In thissense, the current invention discloses that it is possible to achieve awell functioning battery with very low porosity, much below the criticalporosity.

To summarize: one aspect of the present invention is to provide a highdensity electrode which has a low porosity much below of the criticalporosity. Another aspect of the invention is to increase the amount of x(extracted Lithium at full charge per g of cathode). By utilizing theseaspects the safety of the Li rechargeable battery is improved becausenot enough electrolyte is present to allow for a completed CCE reaction.

The bimodal approach of embodiments of the present invention yields highcapacity and low porosity electrodes. HLM has a low electrode density,low ionic transport and also very low electronic conductivity. Byamending these parameters by mixing HLM with a different large particlepowder, a dense cathode powder is obtained. The LiCoO₂ or LiNiO₂ basedmaterials consist of relatively large (>10 μm) particles, and thenano-structured HLM occupies the voids, resulting in a high electrodedensity.

In dynamic cell balancing the irreversible capacity of both anode andcathode are to be compared. The irreversible capacity is the 1^(st)charge inefficiency Qirr={Q(Charge)−Q(Discharge)} divided by Q(Charge).The highest cell capacity is achieved if the irreversible capacities arebalanced. If the anode shows a much larger irreversible capacity thanthe cathode then a lower cell capacity is achieved because not all ofthe cathode capacity is utilized. On the one hand, the high voltageLiCoO₂ based cathodes of the present invention have very lowirreversible capacity. Even at relatively high rate the value is lessthan 5% which is much less than the value of typical anodes. On theother hand, the HLM type cathode materials have a larger irreversiblecapacity, which dramatically increases with the discharge rate, and atfast rate the irreversible capacity by far exceeds the value for typicalanodes. It has been found that a mixture of high voltage stable LiCoO₂and HLM can allow improving the cell balancing as well as dynamic cellbalancing, thus the capacity of the cell can be increased. The electrodecontains large, dense LiCoO₂ based particles. The LiCoO₂ used in theinvention has high bulk lithium transport rate and can be cycled in astable manner at high voltage 4.35V) in full cells. The high voltagestability allows the addition of low cobalt high voltage—high capacitycathode materials—i.e. the Co doped HLM materials—to the cathode.

The LiCoO₂ forms an ideal framework to add the HLM high voltage—highcapacity but low power cathode materials. As a result electrode densityincreases (yielding higher capacity) and porosity decreases (yielding abetter safety). The LiCoO₂ particles create a fast path for Li diffusionacross the electrode, partially replacing the liquid electrolyte. Thesubmicron sized cathode material does itself not allow to obtainmechanically stable relatively thick electrodes without using largeamounts of binder. However, the LiCoO₂ based framework suppliesstability. By mixing with the large particle dense LiCoO₂ mechanicallystable electrodes can be obtained. The high cathode voltage allowsextracting more lithium per cathode volume. This—relatively to thecapacity of the full cell—decreases the amount of electrolyte presentper unit of capacity. Thus a full combustion of all electrolyte deliversless heat per unit capacity. As a result the safety per unit capacity isimproved.

In embodiments of the present invention, cathode materials arecharacterized by

on the one hand:

-   (1) large dense particles    -   (a) to achieve a high electrode density with low porosity, and    -   (b) to enable a short solid diffusion path across the electrode,-   (2) excellent high voltage stability (so that it can be cycled in    the same voltage window as HLM), and-   (3) high ionic transport rate.

And on the other hand: agglomerates consisting of primary particles thatconsist of nano-structured or submicron-sized high capacity cathodematerial such as HLM, to achieve an electrode with very high electrodedensity in mixtures that can be used in a high voltage battery.

The invention is further illustrated in the following examples:

Example 1: Importance of Cobalt Doping of HLM

A major problem to cycle a mixture of large particles LCO and smallerparticles HLM is the high voltage which is required during first chargeof HLM. A typical value is 4.8V, but with each 10 mV that this voltagecan be lowered the undesired side effects—e.g. the high voltageelectrolyte decomposition—will become less severe. The current exampleshows that the voltage of the HLM can be lowered. Increasing the Codoping level causes a desired decrease of the charge voltage. Otherwise,too much Co doping is not desired because it reduces the reversiblecapacity.

3 MOOH precursors are prepared by precipitating a M-NO₃ solution with aNaOH solution in the presence of ammonia. The cobalt content of thesolutions is varied from 0 to 8.3 to 16.7 wt %. Table 1.1 lists the ICPanalysis of the 3 precursors. The composition is very near to thetargeted value. Of each precursor 3 samples with different Li:M ratioare prepared. Each precursor is mixed with Li₂CO₃ and cooked at 800° C.in air for 8 hours. The target Li:M value is calculated as follows:assuming a composition of the final sample Li_(1+x)M_(1−x)O₂ where allMn is tetravalent, all Co is 3-valent and Ni can be 3- or 2-valent. Thetarget Li:M ratio is given by Li:M=(1+x)/(1−x) where x is calculated togive

(a) all Ni is 2-valent

(b) average Ni valence state is 2.5 and

(c) all Ni is 3-valent.

The first charge voltage profile is obtained by coin cell testing. Thecharge rate is 80 mA/g upto 4.8V. In the following the best results(highest capacity of each transition metal composition) are listed. FIG.1 illustrates the voltage profile (capacity in mAh/g of first charge asfunction of charge voltage V) of HLM with (A) 11 mol %, (B) 5 mol % and(C) 0 mol % Co. With increasing Co doping we observe a lowering of thefirst cycle charge voltage in the plateau region. The decrease isapprox. 60 mV per mol % of Co doping. The example demonstrates that HLM,when mixed with large LCO based material, is preferably doped withcobalt. Preferred doping range is 5-20 mol %. The cobalt doping reducesthe necessary charge voltage for FILM, so the LCO and HLM charge voltageprofiles matches better.

TABLE 1.1 Composition of Co containing HLM precursors Ni Co Mn Sample IDwt % wt % wt % Metal composition PHLM-011 10.87 11.04 40.36 Mn_(0.664)Ni_(0.169)Co_(0.167) PHLM-010 12.75 4.96 40.45Mn_(0.710)Ni_(0.209)Co_(0.081) PHLM-012 15.61 0.092 43.48Mn_(0.747)Ni_(0.251)Co_(0.001)

Example 2: Activation of Cobalt Doped HLM

A mixed transition metal hydroxide is obtained by conventionalprecipitation (metal sulfate with NaOH in the presence of ammonia). Themetal composition is M=Ni_(0.22)Mn_(0.66)Co_(0.11). The average particlesize (D50 of the PSD) is about 8 μm. The precursor is relatively dense(tap density=1.36 g/cm³). A final lithium metal oxide (HLM type) isprepared by mixing with Li₂CO₃ (Li:M=1.57, corresponding to 50% of theNi being 2-valent and 50% being 3-valent, assuming all Co is 3-valentand all Mn tetravalent). A high sintering temperature of 1000° C. isapplied resulting in relative dense particles with relatively smallsurface area (0.65 m²/g). SEM shows that primary crystallites areranging from about 0.2 to 0.5 μm in size. Typical secondary oragglomerated particles have sizes of about 5-10 μm.

Such a morphology is not preferred for obtaining a high electrochemicalperformance for HLM. Good performance is achieved after much softersintering resulting in significant significantly smaller crystallites.Typically also better performance is achieved in the case of smallerparticle sized precursors. However, for experimental purposes, afterthese strong sintering conditions particular electrochemical performanceissues of HLM are clearly detected. After strong sintering theactivation typically requires high voltage and several cycles. Wespeculate that the activation is related to an electrochemical grinding.Large particles have severe rate performance issues, so a large increaseof capacity as a result of electrochemical grinding is observed. In thecase of cobalt doping however, activation is no problem, as will beshown in this Example.

Coin cells are prepared by known standard methods. Coin cell testinginvolves 3 different types of activation (different voltages, and 1 or10 cycles), followed by a test for rate performance and cycle stability.The cycling schedule details are given in Table 2.1 below. Independentlyof the type of activation, the tests for rate performance and cyclingstability give identical results. Very tiny differences even indicatethat lower voltage and less activation cycles gives an advantage!

We conclude that in the case of Co doped HLM:

1) 1 activation cycle is sufficient, and

2) 4.6V is sufficient as activation voltage.

Details of the coin cell testing are summarized in FIGS. 2A-C,illustrating:

(A): Rate and stability after single cycle activation at 4.6V (N=1,V1=4.6V)

(B): Rate and stability after single cycle activation at 4.8V (N=1,V1=4.8V)

(C): Rate and stability after 10 cycles activation at 4.6V (N=10,V1=4.6V).

For each of FIGS. 2(A), 2(B) and 2(C) the left figure shows cycles 2-7(from right to left), the right figure cycles 8, 9, 32 & 33 (from rightto left).

A similar experiment is performed for a Co doped HLM prepared at lowertemperature from a fine particle precursor. The surface area is muchlarger, about 5 m²/g. It is confirmed that also for such HLM cathodeperformance after a single activation at 4.6V gives the bestperformance.

TABLE 2.1 Cycle schedule definition Charge Discharge Voltage, Voltage,Cycle current Type current Type Purpose 1 to N V1, 0.1 C CC 2.0 V, 0.1 CCC Activation of HLM 1 + N 4.6 V, CC/CV 2.0 V, CC Rate to 0.25 C/0.05 C0.2, 0.5, 1, performance 6 + N 2, 3 C  7 + N, 4.6 V, CC/CV 2.0 V, 0.1 CCC Slow reference 31 + N  0.25 C/0.1 C  cycle before/after stabilitytest  8 + N, 4.6 V, CC/CV 2.0 V, 1 C   CC Fast reference 32 + N  0.25C/0.1 C  cycle before/after stability test 9 + N 4.6 V, CC 2.0 V, 0.5 CCC Stability test to 0.25 C 30 + N  (CC = constant current, CC/CV =constant current/constant voltage)

Example 3: Voltage Compatibility of LCO and HLM

HLM type materials are considered to require special formation cycles. Atypical formation schedule is an activation either at 4.8V for one ormore cycles. 4.8V is a very high voltage; when HLM is mixed with othercathode materials a 4.8V cycle can damage other cathode components.Alternatively it is often recommended to gradually increase the voltageduring several formation cycles. Implementing such complex specialformation at mass production requires severe investment and makes theapplication of HLM difficult in real cells. Example 2 showed that forHLM containing cobalt a single activation cycle at 4.6V is sufficient.Cycling HLM at 4.5V gives a high capacity. (At 4.5V the capacity is onlyabout 5% less than 4.6V capacity.) Example 3 will show that theseformation conditions of Example 2 are compatible with the “high voltageLiCoO₂” discussed before. This is an important requirement if HLM andLCO cathode materials are used together. Example 3 proves that a mixtureof HLM with high voltage LiCoO₂ can cycle well in real batteries.

A high voltage stable LiCoO₂ based cathode is obtained from a pilotplant, according to co-pending application WO2012-171780. The Li:Coratio is 1.0 and the electrical conductivity of the LCO powder is below10⁻⁴ S/cm (1.2*10⁻⁵ S/cm). The conductivity is measured under a pressureof 63.7 MPa at room temperature. The LCO contains Mg (1 wt %). Themajority of particles have a large size of 20 μm. The average particlesize (D50 of the PSD) is 18 μm. The surface area of the LiCoO₂ basedcathode is 0.18 m²/g.

Coin cell testing is performed according to different schedules namedV3.1-V3.2. V3.1 is the charge voltage of cycle 1 whereas V3.2=4.5V,being the charge voltage of cycles 2-32. Table 3.1 summarizes theschedules, and Table 3.2 summarizes the obtained results. The tableshows the capacity loss and energy loss (capacity×average voltage) per100 cycles (in percent) extrapolated from cycle 7 and 31 (for 1 C rate)and cycle 8 and 32 (for 0.1 C rate) The data in Table 3.2 show thatV3.1=4.3V gives an excellent cycle stability. A similar stability isobtained with V3.1=4.6V. V3.1=4.7V still shows acceptable cyclestability (but less than V3.1=4.6V) whereas V3.1=4.8V shows somedeterioration.

From Table 3.2 we can conclude that high voltage LiCoO₂ used in thepresent invention is compatible with HLM. High voltage stable LiCoO₂tolerates an activation cycle at ≥4.6V, and allows for cycling withoutsignificant capacity fade at 4.5V. Example 2 demonstrated similarelectrochemical testing for HLM. Example 3 demonstrates that theelectrochemical properties of high voltage stable LiCoO₂ is compatiblewith HLM type voltage range, particularly if the HLM contains Coallowing to reduce the voltage V1 of a single activation cycle to 4.6V.

TABLE 3.1 Cycling schedule V 3.1-V 3.2 (1 C = 160 mA/g) Charge DischargeVoltage, Voltage, Cycle current Type current Type Purpose 1 V3.1, 0.1 CCC 2.0 V, 0.1 C CC Activation of HLM 2-6 V3.2, CC/ 2.0 V, CC Rateperformance  0.25 C/0.05 C CV 0.2, 0.5, 1, 2, 3 C 7, 31 V3.2, CC/ 2.0 V,0.1 C CC Slow reference cycle 0.25 C/0.1 C CV before/after stabilitytest 8, 32 V3.2, CC/ 2.0 V, 1 C CC Fast reference cycle 0.25 C/0.1 C CVbefore/after stability test 10- V3.2, 0.25 C CC 2.0 V, 0.5 C CCStability test 31

TABLE 3.2 Coin cell testing results for high voltage stable LiCoO₂ QQ_(4.5V) Q_(4.5V) Voltage DQ Qirr (0.1 C) Q (1 C) E (0.1 C) E (1 C) (0.1C) (1 C) V3.2 = 4.5 V mAh/g % %/100 %/100 %/100 %/100 mAh/g mAh/g V3.1 =4.3 V 161.0 1.1 3.1 6.9 3.5 9.0 194.5 189.6 V3.1 = 4.6 V 228.8 1.3 3.47.6 3.9 10.0 194.1 188.4 V3.1 = 4.7 V 250.9 1.6 4.7 11.6 5.8 14.7 193.2187.2 V3.1 = 4.8 V 255.9 1.7 5.8 15.6 7.2 19.5 193.0 187.0

The following definitions are used for data analysis: (Q: capacity, DQ:Discharge Capacity, CQ: Charge Capacity)

The discharge capacity QD is measured during the first cycle in the4.3-3.0 V range at 0.1 C (in mAh/g). Irreversible capacity Qirr is(QC1−QD1)/QC1 (in %).

Fade rate (0.1 C) per 100 cycles, for capacity Q (0.1 C):(1−QD31/QD7)*100/23.

Fade rate (1 C) per 100 cycles, for capacity Q (1 C):(1−QD32/QD8)*100/23.

Energy fade E (0.1 C)& E (1 C): instead of discharge capacity QD thedischarge energy (capacity×average discharge voltage) is used.

Q_(4.5V) (0.1 C) and Q_(4.5V) (1 C): discharge capacity of cycle 7 (at0.1 C) and of cycle 8 (at 1 C)

FIGS. 3A-C illustrates the electrochemical testing of high voltagestable LiCoO₂ for:

(A) first cycle 4.3V

(B) first cycle 4.6V

(C) first cycle 4.8V.

For each of FIGS. 3(A), 3(B) and 3(C) the left figure shows cycles 1-6(from right to left), the middle cycles 7, 8, 31 & 32 (from right toleft). The right figures show the fade rate: capacity in mAh/g againstcycle number (charge: small circles, discharge: bigger circles).

Example 4: Voltage Compatibility of LCO and HLM, and Compatibility ofHLM-LCO Mixtures with Anodes

A mixed transition metal hydroxide is obtained by conventionalprecipitation (metal sulfate with NaOH in the presence of ammonia). Themetal composition is M=Ni_(0.22)Mn_(0.66)Co_(0.11). The average particlesize (D50 of the PSD) is about 3-4 μm, the precursor is consisting ofrelatively loose agglomerated sub-micrometer particles. The precursorhas a low tap density of about 0.6 g/cm³. Final HLM type cathodematerial is prepared by mixing the precursor with Li₂CO₃ (Li: transitionmetal blend ratio=1.442, at this ratio it is assumed that Ni isdivalent) and firing at 800° C. for 10 hours. The chemical formula ofthe final product is estimated asLi_(1.181)Ni_(0.182)Mn_(0.546)Co_(0.091)O₂. The surface area of thefinal sample is 4.5 m²/g. The morphology is “fluffy”, meaning that thepowder consists of loose agglomerates of sub-micrometer particles. Thesesubmicrometer particles have a size about 100 nm.

A high voltage stable LiCoO₂ based cathode is obtained from our pilotplant (made according to the process in WO2012-171780). The Li:Co ratiois 1.00 and the electrical conductivity of the LCO powder is below 10⁻⁴S/cm². The LCO contains Mg (1 wt %). The majority of particles have alarge size of 25 μm. The average particle size (D50 of the PSD) is 22μm. The surface area of the LiCoO₂ based cathode is below 0.15 m²/g.

The LiCoO₂ and the HLM powder are mixed using different weight ratios:

Sample LCO:HLM

S4A: 90:10;

S4B: 75:25;

S4C: 50:50.

Coin cells are prepared and tested by the schedule used in Example 2(A)for testing HLM, here with is 1 activation cycle (N=1) at V1=4.6Vfollowed by rate test and stability test at V2=4.5V (instead of 4.6V inTable 2.1). Table 4.1 summarizes the obtained results.

TABLE 4.1 Coin cell testing of mixtures of HLM and high voltage stableLCO. Coin Cell Schedule: V1 = 4.6V, V2 = 4.5V Fading rate (per 100 cyc)DC Q Qirr DC Q 1 C Rate 3 C Rate Capacity Energy Sample mAh/g % 0.1 C%/0.1 C %/0.1 C 0.1 C 1 C 0.1 C 1 C S4A 234.6 3.8 196.8 95.6 90.6 6.512.3 7.5 14.1 S4B 232.5 5.9 194.8 93.8 88.3 10.1 16.1 12.1 19.7 S4C239.3 7.6 201.2 90.5 84.2 14.5 19.0 17.4 25.0

FIGS. 4A, 4B, and 4C summarize the coin cell testing details. (A=S4A, .. . ). For each of FIGS. 4(A), 4(B) and 4(C) the left figure showscycles 2-7 (from right to left), the middle cycles 8, 9, 32 & 33 (fromright to left). The right figures show the fade rate: capacity in mAh/gagainst cycle number (charge: small circles, discharge: bigger circles).

Usually, the properties of 100% LCO do not match real anodes well. 100%LiCoO₂ has a too high charge efficiency (near to 98.5%) and a high rateperformance at 3 C rate (91.5%).

At the same time 100% HLM, compared with typical anodes, has a lowcharge efficiency (<90%) and a low rate performance. Therefore also HLMdoes not match well with a typical anode. In the invention, mixturesmatch well with real anodes, and a sufficient high cycling stability isachieved. The region between Sample S4A and S4B corresponding to 10-25%HLM shows particular promising properties since an optimum highestdensity coincides with higher capacity, slightly lower rate andincreased irreversible capacity when comparing with 100% LiCoO₂. TheExample illustrates that the voltage window of (cobalt doped) HLM andLiCoO₂ are compatible and that a mixture of LiCoO₂ and HLM matches muchbetter to real anodes in terms of charge efficiency and rate performancethan either HLM or LiCoO₂. At 4.5V about or more than 195 mAh/g capacitycan be achieved.

Example 5: LiCoO₂ Based Electrodes with Little or no Liquid Electrolyte

A high voltage stable LiCoO₂ is prepared by a double firing, as inWO2012-171780. The D50 of the particle size distribution is 50 μm whichis consistent with the result of a PSD measurement. The particles aredense, with a pressed density of at least 4 g/cm³ and a BET value ≤0.2m²/g. FIG. 5 shows a typical SEM micrograph of the LiCoO₂. Left: 1000×magnification. Right: relatively small particles located on surface of alarger 40 μm particle in larger magnification (5000×). Besides of a verysmall amount of fine particles the large particles are 100% dense withno open porosity.

Coin cells are prepared, wherein the electrode consists of 96 wt % ofactive material. The electrode loading is about 6 mg/cm². Table 5.1lists the coin cell test results. FIG. 6 shows the electrochemicaltesting results: rate performance in thin electrode configuration: leftFigure: from right to left Cycles 1 to 6, with corresponding rate: C/10,1C, 5C, 10C, 15C, 20C; right figure: rate “% versus 0.1C” against “Crate” (in hour). A typical discharge rate for lithium batteries is a 1Crate. The high density, large particle LiCoO₂ demonstrate 92% at 1Crate, which is sufficient for practical applications. Alternatively,cells with 12 mg/cm² load were cycled with V1=4.3, V2=4.5V scheduledescribed in Example 3 and showed very high cycle stability. Results aresummarized in Table 5.2 and FIG. 7.

TABLE 5.1 Results of coin cell testing (4.4-3.0 V, 1 C = 160 mA/g) Cy1(Charge) Cy 1 (Disch) Cy 2 (DisCh) Cy 3 (DisCh) Cy 4 (DisCh) 0.1 C,mAh/g 0.1 C, mAh/g 1.0 C, % 5.0 C, % 10 C, % 180.7 175.7 91.8 82.4 76.2The % value of the rate performance is obtained by dividing thedischarge capacity of the cycle by the discharge capacity at a rate of0.1 C.

TABLE 5.2 Results of coin cell testing (see schedule of Ex. 3, with V3.1= 4.3 V, V3.2 = 4.5 V, 1 C = 160 mA/g) rate Fade Fade E Fade E FadeQ(C/10) Qirr (1 C) (0.1 C) (1 C) (0.1 C) (1 C) mAh/g % % %/100 %/100%/100 %/100 158.6 4.17 90.5 8.2 8.4 8.2 11.4

FIG. 7 gives a summary of the coin cell testing results (cyclestability) of 50 μm LCO particles. The left figure shows cycles 1-6(from right to left), the middle cycles 7, 8, 31 & 32 (from right toleft), following the schedule used in Example3, with V3.1=4.3V,V3.2=4.5V. The right figures show the fade rate: capacity in mAh/gagainst cycle number (charge: small circles, discharge: bigger circles).

After the test the cell is disassembled and the particles in theelectrode are analyzed by FESEM and XRD. FESEM shows that the particlesremain dense, they do not shatter into pieces. XRD shows similar narrowpeaks as before testing, proving that the crystal structure does notdeteriorate. In general, as the particle size of dense compact particlesincreases, the rate performance of these materials decreases. This isbecause larger particles have a longer Li diffusion path length.Comparing Li diffusion path lengths is a useful tool to study theexpected rate performance for different shaped particles or objects. Apossible definition of the path length (diffusion length) is the averageshortest distance of each Li atom in the particle to the surface. We canestimate the average solid diffusion path length of the large PSD LCO.If we assume that all particles are spheres of 50 μm diameter then thepath length is the average of the distance from the surface R-rmultiplied with the volume fraction. At position r the volume tractionis 4πr²/Vol where Vol=4/3πR³. Integrating over the sphere all distancesmultiplied by the volume fraction gives

${\frac{3}{R^{3}}{\int_{0}^{R = {25\mspace{11mu}{µm}}}{{\mathbb{d}{r\left( {R - r} \right)}}r^{2}}}} = {\left( \frac{R}{4} \right) = {6.25\mspace{14mu}{µm}}}$

A flat, dense (plate type) electrode of 12.5 μm thickness has the sameaverage diffusion length, so it should exhibit roughly a similar rateperformance. With a density of 5.05 g/cm³ for LiCoO₂ the cathode loadingof the plate type electrode is 6.3 mg. Hence we estimate the rateperformance of a liquid free electrode plate as >90% at 1C rate for 6.3mg/cm² loading. 6.3 mg/cm² loading is less than the 15 mg/cm² loadingfor typical state of art lithium batteries. Otherwise, the powderrequirement for a solid cathode can be less. We can use the diffusionlength L defined as L²=2d Dt by using the relation

$t \propto \frac{L^{2}}{D}$where L is the diffusion length and D the diffusion constant to estimatethe change of rate performance for different thick electrodes if we knowthe rate for a given thickness. For a 20 mg/cm² dense electrode plateusing the 10C rate performance of Table 5.1 we obtain a 1C rateperformance of about 75%. In theory even thicker plate electrodes couldbe utilized because the solid diffusion does not show the rate limitingeffects (electrolyte shut-down) known for binary electrolytes.

To summarize: The solid diffusion constant of high voltage LiCoO₂ issufficiently high to allow for liquid electrolyte free electrodes. Inmixed electrodes (for example HLM+LiCoO₂) the large LiCoO₂ particles canact as solid diffusion short-cuts between 2 regions with HLM particles,thus dramatically reducing the required amount of liquid electrolyte.Such electrodes will have extremely good safety properties. The exampleclearly demonstrates that electrodes with very low or even zero contentof liquid electrolyte can have sufficient power.

Example 6: Low Porosity Electrode

The example will demonstrate low porosity electrodes. The large particleLiCoO₂ (50 μm size) of Example 5 and the HLM of Example 4 are used.Slurries for coating are prepared from mixtures of LCO and HLM. Asconductive additive super P is used, as binder and solvent a 5% PVDF inNMP solution is used. Table 6.1 summarizes the slurry composition. Apart of the slurries are coated on Al foil as a thick film, dried,pealed off, and grinded to obtain electrode powder. The electrode powderdensity is measured on pressed pellets. Generally the electrode powderdensity is a very good approximation for a real electrode density whichcan be obtained with the powder. The diameter of the pressed pellets is1.311 cm and the mass is 3.0 g, the applied force is 207 MPa. Theobtained pellet density values are 4.153 g/cm³ (5% FILM); 4.036 g/cm³(10% HLM) and 4.123 g/cm³ (15% HLM). Use of the true density of LiCoO₂(5.05 g/cm³), HLM (4.251 g/cm³), PVDF (1.7 g/cm³) and carbon (2 g/cm³)allows to calculate the porosity. The result is 11.3%; 11.6% and 7.6%.These data show that electrodes with very low porosities can beachieved. The porosities are much less than the critical porositiescalculated below in Example 8. Typical electrode porosities incommercial cells are >12%, often 15-20%.

The remaining slurry is used to coat electrodes of high loading followedby drying and compacting. The active cathode load is 58, 50 and 45mg/cm². These loads are very high, 3-4 times larger than normalelectrodes in commercial cells. Coin cell testing shows full capacitywithin 1-2% of expected value when tested at C/20 rate. The referencevalue is obtained for 3.0-4.3V with 12 mg/cm² electrodes at C/10 rate,1C=160 mA/g and is similar to the theoretically expected value (averageof LCO and HLM capacity). During further cycling of the cells with thickelectrodes the electrodes pealed peeled off, known current coatingtechnology is not optimized to obtain stable cycling with very highload. However the Example demonstrates that very thick electrodes withlow porosity, promising excellent safety can be reversible cycled.

TABLE 6.1 Electrode making using mixtures of fluffy HLM and 50 μm LCOHLM PVDF Carbon HLM LCO Solid active % sol g g g g % % 5 12.377 0.6191.5 28.5 72.7 96.04 10 15.231 0.762 3 27 68.5 95.17 15 15.115 0.756 3.7521.25 64.9 94.30

Example 7: High Safety if Less Electrolyte Present

This example demonstrates the absence of a major exothermic reaction ofdelithiated cathodes when no electrolyte is present. For this experimentcommercial LiMO₂ with M=Ni_(0.5)Mn_(0.3)Co_(0.2) is used as modelmaterial. Coin cells (with small positive electrodes (cathode) and Limetal anode) are prepared and charged to 4.3V. Cathode active materialweight is about 3 mg. The coin cells are opened, thereby payingattention not to short circuit the cell. The electrode is washed in DMC,then dried at 120° C. for 10 min in air.

The dried electrode is inserted in a stainless steel DSC cell and adefined amount of electrolyte is added. In this Example, 1.1 mgelectrolyte is added in one experiment, in the other experiment noelectrolyte is added. Then the cells are hermetically sealed. Theexothermic heat is measured during heating to 350° C. at a rate of 5K/min. The mass of the DSC cell is checked before and after measurementto ensure that the DSC cell does not leak.

FIG. 8 compares the result of DSC measurements of charged electrodes (A)without and (B) with 1.16 mg of electrolyte added. The measurement showsthe typical strong exothermic event at about 290° C. where chargedcathode and electrolyte start a violent reaction. However, in theabsence of electrolyte almost no heat evolution is observed. At firstglance this result—the absence of an exothermic event—is a surprisesince delithiated Li_(0.4)MO₂ is thermodynamically highly unstable. Thisis because during the charging process (reaction 1 below) the extractionof Li requires a large amount of energy (at 4.3V about 3.7V×170 mAh/g).Then, during heating the cathode collapses and releases oxygen (reaction2 below). At first glance it can be expect that the reaction 2) fromunstable compounds to stable compounds is strongly exothermic. So theresult (small exothermic heat for 2)) appears surprising. But if we lookat the whole thermodynamic cycle by adding reaction 3) we can understandthat reaction 2) does not evolve much exothermic heat.Charge: stable→unstable: LiMO₂→Li_(0.4)MO₂+0.6Li  1)Cathode collapse: unstable→stable:Li_(0.4)MO₂→0.4LiMO₂+0.3M₂O₃+0.15O₂  2)Complete the cycle: 0.4LiMO₂+0.3M₂O₃+0.15O₂+0.6Li→LiMO₂  3)

We can split reaction 3) into 2 reactions:0.6Li+0.15O₂→0.3Li₂O (strong exotherm)  3a)0.3Li₂O+0.3M₂O₃→0.6LiMO₂  3b)

In these reactions only 3a) is strongly exotherm (Li burns with theevolution of lots of heat). None of the other reactions 1), 3b) isstrongly endotherm: In reaction 1) ΔH₁ is near zero because a batterydoes not change much in temperature when charged or discharged. Inreaction 3b) ΔH_(3b) is near zero because generally the creation ofdouble oxides from simple oxides does not create or consume much heat.Therefore reaction 2) cannot be strongly exotherm, because in athermodynamic cycle the sum of the formation enthalpies adds up to zero:0=ΔH₁+ΔH₂+ΔH_(3a)ΔH_(3b). If however, reaction 2) is not strongexotherm, then no heat evolves during cathode collapse and the batterydoes not self-heat and does not go into thermal runaway. In the presenceof electrolyte however, the situation is different, because the cathodecollapse occurs simultaneously with the very exothermic combustion ofelectrolyte. It can be concluded that reducing the electrolyte: activemass ratio allows to improve the safety of real batteries.

Example 8: Improved Safety by Lowering the Electrolyte: Cathode Ratio

Charged Li batteries are potentially unsafe. In the worst case scenariothe delithiated cathode and the lithiated anode react with theelectrolyte, causing a thermal runaway. The major contribution to theexothermic heat is the electrolyte oxidation: the charged cathode canrelease oxygen which combusts the electrolyte. If we assume that x inequation (4) (of the CCE reaction described before) is 0.5, then 1 molcharged Li_(0.5)CoO₂ can combust 0.1 mol C₃H₄O₃ (ethylene carbonate (EC)based electrolyte). We can estimate how much electrolyte can becombusted by the oxygen released from the cathode. We assume x=0.5, andfor simplicity we use EC to represent the electrolyte.

-   -   1 mol LiCoO₂=97.9 g    -   0.1 mol C₃H₄O₃=8.8 g

The electrolyte: cathode mass ratio which allows for 100% combustion isabout 9 percent (8.8/97.9≅0.09, theoretically for x=1 this would be18%). The result is that the safety of a battery can be improved if muchless than about 9% by weight of electrolyte relative to the activecathode is added to a battery. If less electrolyte is present, then lesselectrolyte combusts and the thermal safety will improve.

The electrolyte fills the pores in the electrodes. The pores must befilled with electrolyte otherwise the battery will have a poorperformance. For simplicity—lets assume that ½ of the electrolyte is inthe pores of the cathode (this is achieved for example if cathode andanode have the same thickness and same porosity). Lets furthermoreassume that the separator is thin, so we can neglect its porosity, andlets assume that the electrolyte exactly fills the porosity, meaning no“leftover” electrolyte is present in the battery after assembly. Thecritical porosity is the porosity which—when filled withelectrolyte—corresponds to a ratio of electrolyte to cathode activematerial of 9 wt %. Using these assumptions together with the densitiesof cathode and electrolyte we can estimate a critical porosity asfollows:

-   -   LiCoO₂: Density 5.05 g/cm³    -   Typical electrode composition: LiCoO₂: 96 wt %        -   (2% Binder (1.77 g/cm³), 2% carbon (2 g/cm³))→electrode            theoretical density (0% porosity)=4.92 g/cm³    -   C₃H₄O₃: Density 1.32 g/cm³    -   Electrolyte: 9% (by weight, per mass of cathode) present in the        battery, of those 9% ½ (=4.5%) present in the cathode.

Using these data allows to calculate the critical porosity as 13.7% (involume), knowing that vol1=vol electrode=1/(0.96*4.92)=0.214; vol2=volelectrolyte=0.045/1.32=0.034; and porosity=vol2/(vol1+vol2). Thecritical porosity increases with charge voltage. The high voltage stablecathodes allow for reversible cycling at >4.5V. At 4.5V we achieve areversible capacity of >185 mAh/g (the total charge being 280mAh pergram when all Li is extracted). This corresponds to x>0.675 in the 4.5Vcharged Li_(1−x)CoO₂, corresponding to a critical porosity of 20.5%. Ifthe real cathode porosity is further decreased or if x is increased(more lithium extracted at full charge by charging at a higher voltage)then we can assume that the safety of a battery having a certaincapacity improves. The example shows that electrodes with small porosityallow to reduce the electrolyte sufficiently to achieve improved safety.

Example 9

A LiCoO₂ with an average particle size (D50) in excess of 100 μm isprepared by using a large excess of lithium and sintering at hightemperature. After sintering the excess Li is removed, resulting in astoichiometric LiCoO₂, by performing the following steps:

firing a blend of Li₂CO₃ and Co₃O₄ (mol ratio Li:Co=1.2) at 990° C. for12 h

removing excess Li₂CO₃ by washing,

followed by adding of more Co₃O₄ (about 6% Co per mol LiCoO2), and

re-firing at 950° C. The particles of the final sample are dense “rock”shaped.

FIG. 9 shows a micrograph of the particles of the obtained powder.

A Li and manganese rich cathode material—referred to as HLM—withcomposition Li_(1+x)M_(1−x)O₂ is prepared from a suitable MCO₃ precursorby mixing with Li₂CO₃ and firing in air at 800° C. The final compositionhas a Li:M ratio of about 1.42 and a transition metal compositionM=Mn_(0.67)Ni_(0.22)Co_(0.11). FIG. 10 shows a micrograph of theparticles of the obtained powder. Note that the magnification in FIG. 10is 10× that of FIG. 9.

The LiCoO₂ and HLM cathode powders are mixed. 3 mixtures are preparedcontaining 5, 10 and 20% by mass of HLM powder, labeled M05, M10, M15.The powder density is measured by compacting the pellets to a Density 1,then, after relaxing the pressure a Density 2 is measured. A very highdensity of 4.17 g/cm³ is measured for M10. This high density indicatesthat a very low porosity of 5 vol % or so can be achieved in realelectrodes.

The mixtures are tested in Li coin cells. Coin cell are tested at a rateof C/20 (corresponding to a rate of 8 mA/g) between 2.0 and 4.6V. Cycle2 is at C/10 rate (16 mA/g) between the same voltage limits. A very highreversible capacity is achieved, proving that even the very large anddense LiCoO₂ particles can cycle well. Table 9 summarizes the results.FIG. 11 shows the obtained voltage profiles.

TABLE 9 LiCoO₂:HLM Q discharge Density1 Density2 Sample mass ratio mAh/gg/cm³ g/cm³ M05 95:5  227.4 mAh/g 4.30 4.17 M10 90:10 230.7 mAh/g 4.174.07 M20 80:20 237.0 mAh/g 3.97 3.89

Example 10

This example will demonstrate the improved safety of a cathode when lessthat than the critical amount of electrolyte is present. The examplepredicts that a cathode with a very low porosity—which only allows forless than the critical amount of electrolyte to be present—will provideimproved safety.

The safety of sample M10 of Example 9 is estimated by a DSC measurement:5 coin cells are prepared and charged at 25° C. to 4.5V at C/10 rate (16mA/g). The obtained capacities are 197.2-197.8 mAh/g. The cells aredisassembled directly after reaching the target voltage. The electrodesare washed in DMC to remove the electrolyte. After drying, smallelectrode discs with 3 mg of active material are inserted into DSCcells. 3 different types of DSC cells are prepared:

Cell type 1) No electrolyte is added, the cell is just crimped,

Cell type 2) about 2.6 mg of an electrolyte (ED/EMC) diluted 1:10 by DMCis added. After a few moments most of the DMC is evaporated, and thecell is crimped,

Cell type 3) about 2.6 mg of electrolyte is added and the cell iscrimped.

In this way DSC cells with a electrolyte:cathode ratio of

-   (1) zero (being much lower than the critical ratio),-   (2) 0.08—being less than the critical ratio—and-   (3) 0.46—by far exceeding the critical ratio—are obtained.

The heat evolution is measured during heating at a rate of 5 K/min to350° C.

Table 10 summarizes the obtained results. FIG. 12 shows the obtained DSCheat profiles. Clearly, as the amount of electrolyte decreases below thecritical ratio the evolved heat decreases, thus the safety of a batterywith a small amount of electrolyte will be high.

TABLE 10 0% electrolyte 8% electrolyte 46% electrolyte Integrated heat(kJ/g) 0.136 0.725 1.551 (1.601) cathode

It should be clear that the conclusions of the different examplesembodying the invention are valid for undoped and doped LCO, the dopantsbeing for example Mg, Ti, Fe, Cu, Ca, Ba, Y, Sn, Sb, Na, Zn, Zr, Si, Er,Nd and Nb.

The invention claimed is:
 1. A bimodal lithium transition metal oxidebased powder for a rechargeable battery, comprising: a first lithiumtransition metal oxide based powder having an average particle size(D50) of at least 15 μm and comprising a material A, having a layeredcrystal structure comprising the elements Li, a metal M and oxygen,wherein the Li content is stoichiometrically controlled, wherein themetal M has a formula M=Co_(1−a)M′_(a), with 0≤a≤0.05, wherein M′ is oneor more metals selected from the group consisting of Al, Ga and B, andwherein the first lithium transition metal oxide based powder has anelectrical conductivity of less than 10⁻⁴ S/cm; and a second lithiumtransition metal oxide based powder having the formulaLi_(1+b)N′_(1−b)O₂, wherein 0.10≤b≤0.25, and N′=Ni_(x)Mn_(y)Co_(z)A_(d),wherein 0.15≤x≤0.30, 0.50≤y≤0.75, 0.05<z≤0.15 and 0≤d≤0.10, and whereinA is a dopant; wherein the second powder has an average particle size(D50) of less than 5 μm and wherein the ratio of the D50 value of thefirst powder to the D50 value of the second powder is at least 5:1.
 2. Abimodal lithium transition metal oxide based powder for a rechargeablebattery, comprising: a first lithium transition metal oxide based powderhaving an average particle size (D50) of at least 15 μm and comprising amaterial B, having a core and a surface layer, wherein the corecomprises a layered crystal structure comprising the elements Li, ametal M and oxygen, wherein the Li content is stoichiometricallycontrolled, wherein the metal M has a formula M=Co_(1−a)M′_(a), with0≤a≤0.05, and wherein M′ is one or more metals selected from the groupconsisting of Al, Ga and B; wherein the surface layer comprises amixture of the elements of the core and inorganic N-based oxides,wherein N comprises one or more metals selected from the groupconsisting of Mg, Ti, Fe, Cu, Ca, Ba, Y, Sn, Sb, Na, Zn, Zr, Si, Nb, Mo,Ru, Rh, Pd, Ag, Cd, Sc, Ce, Pr, Nd, Gd, Dy, and Er, and wherein thefirst lithium transition metal oxide based powder has an electricalconductivity of less than 10⁻⁴ S/cm; and a second lithium transitionmetal oxide based powder having the formula Li_(1+b)N′_(1−b)O₂, wherein0.10≤b≤0.25, and N′=Ni_(x)Mn_(y)Co_(z)A_(d), wherein 0.15≤x≤0.30,0.50≤y≤0.75, 0.05≤z≤0.15 and 0≤d≤0.10, and wherein A is a dopant;wherein the second powder has an average particle size (D50) of lessthan 5 μm.
 3. The bimodal lithium transition metal oxide based powder ofclaim 1, wherein the first lithium transition metal oxide based powderhas a surface area measured by BET of less than 0.20 m²/g, and thesecond lithium transition metal oxide based powder has a surface areameasured by BET of at least 0.50 m²/g.
 4. The bimodal lithium transitionmetal oxide based powder of claim 2, wherein the ratio of the weightfraction of the first lithium transition metal oxide based powder to theweight fraction of the second lithium transition metal oxide basedpowder is at least 2:1.
 5. The bimodal lithium transition metal oxidebased powder of claim 2, wherein the first lithium transition metaloxide based powder has an electrical conductivity of less than 10'S/cm,measured under a pressure of 63.7 MPa.
 6. A positive electrode for arechargeable battery, comprising the bimodal lithium transition metaloxide based powder of claim 1, and having a porosity ≤15%.
 7. Arechargeable battery comprising the positive electrode of claim 6,wherein less than 15 vol % of organic electrolyte fills pores of thepositive electrode.
 8. The rechargeable battery of claim 7, wherein thetotal mass of electrolyte per mass of positive electrode is less than15%.
 9. The bimodal lithium transition metal oxide based powder of claim2, wherein the first lithium transition metal oxide based powdercomprises large dense particles having a D50 of at least 15 μm, andwherein the ratio of the D50 value of the first lithium transition metaloxide based powder to the D50 value of the second lithium transitionmetal oxide based powder is at least 3:1.
 10. The bimodal lithiumtransition metal oxide based powder of claim 2, wherein the firstlithium transition metal oxide based powder has a surface area measuredby BET of less than 0.20 m²/g, and the second lithium transition metaloxide based powder has a surface area measured by BET of at least 0.50m²/g.
 11. A positive electrode for a rechargeable battery, comprisingthe bimodal lithium transition metal oxide based powder of claim 2, andhaving a porosity ≤15%.
 12. A rechargeable battery comprising thepositive electrode of claim 11, wherein less than 15 vol % of organicelectrolyte fills pores of the positive electrode.